The invention relates to an ultrafine copper alloy wire and a process for producing the same, and more particularly to an ultrafine copper alloy wire having a diameter of not more than 0.08 mm for use, for example, in electronic equipment, IC testers, and medical ultrasound system, and a process for producing the same.
A reduction in size of electronic equipment, IC testers, medical ultrasound system and the like has led to a demand for a reduction in diameter of electric wires for these types of equipment. In particular, in the case of electric wires for medical ultrasound system, there is a demand for electric wires (cables) which have an increased number of wire cores (micro coaxial cables) while maintaining the outer diameter of conventional electric wires.
An example of a material for conductors of electric wires for medical ultrasound system currently in use in practical applications is a dilute copper alloy comprising an oxygen-free copper (OFC) as a base metal and a very small amount of a metallic element, such as tin, added to the base metal. The dilute copper alloy is melted and cast into a wire rod which is then drawn through a die to a diameter of 0.03 mmxcfx86 to prepare an ultrafine copper alloy wire. This ultrafine copper alloy wire is mainly used as conductors in electric wires for medical ultrasound system.
When an ultrafine copper alloy wire having a smaller diameter (for example, not more than 0.025 mmxcfx86) is formed as a conductor for electric wires from the viewpoint of further reducing the diameter of wire cores for medical ultrasound system, however, excessively low breaking strength of the conductors using the conventional copper alloy causes frequent breaking of wires at the time of wire drawing or standing of the conductors. For this reason, the formation of ultrafine copper alloy wires having a diameter of not more than 0.025 mmxcfx86 using conventional alloys was very difficult.
Thus, ultrafine copper alloy wires having higher tensile strength have been desired. Merely increasing the tensile strength, however, results in lowered electrical conductivity. This had led to a demand for copper alloys having both high tensile strength and high electrical conductivity.
Further, excellent drawability is required for the formation of ultrafine copper alloy wires having a diameter of not more than 0.025 mmxcfx86. When a wire rod is drawn by dicing, the presence of foreign materials having a size of about one-third of the wire diameter in the wire rod poses a problem of wire breaks. Therefore, the amount of foreign materials contained in the wire rod should be reduced to improve the wire drawability.
Detailed analysis of the foreign materials contained in a sample of a broken wire has revealed that the cause of the inclusion of foreign materials in the wire rod is classified roughly into two routes. One of them is inclusions contained in the copper alloy as a base material and the metallic elements as the additive, and peeled pieces produced by the separation of refractories such as SiC, SiO2, and ZrO2, which are components of ceramics and cement used in crucibles employed in melting and/or molds used in casting. The other route is foreign materials externally included during wire drawing. Among these foreign materials, the inclusion of the latter type of foreign materials can be reduced by performing the step of wire drawing in a clean environment.
On the other hand, improving the quality of the base material (improving the purity of substances constituting the base material) is necessary for reducing the amount of the former type of foreign materials (inclusions and peeled pieces). Therefore, when ultrafine wires are formed by wire drawing, very careful attention should be paid so as to avoid the inclusion of foreign materials in steps from melting to wire drawing, and the factor in the inclusion of the foreign material should be minimized.
The invention has been made with a view to solving the above problems of the prior art, and it is an object of the invention to provide an ultrafine copper alloy wire having excellent tensile strength, electrical conductivity, and drawability, and a process for producing the same.
According to the first feature of the invention, there is provided an ultrafine copper alloy wire drawn to a diameter of not more than 0.08 mm, said ultrafine copper alloy wire being formed of an alloy comprising a copper matrix of high purity copper with a total unavoidable impurity content of not more than 10 mass ppm and, contained in the matrix, 0.05 to 0.9 mass % of at least one metallic element selected from the group consisting of tin, indium, silver, antimony, magnesium, aluminum, and boron.
According to the second feature of the invention, there is provided an ultrafine copper alloy wire comprising: a core wire formed of an alloy and drawn to a diameter of not more than 0.08 mm, said alloy comprising a copper matrix of high purity copper with a total unavoidable impurity content of not more than 10 mass ppm and, contained in the matrix, 0.05 to 0.9 mass % of at least one metallic element selected from the group consisting of tin, indium, silver, antimony, magnesium, aluminum, and boron; and, provided on the periphery of the core wire, a tin plating, a silver plating, a nickel plating, a tin-lead solder plating, a tin-copper-bismuth-base plating, or a tin-silver-copper-base lead-free solder plating.
The above constitutions can realize ultrafine copper alloy wires having high tensile strength and high electrical conductivity.
According to the third feature of the invention, there is provided a process for producing an ultrafine copper alloy wire to be drawn to a diameter of not more than 0.08 mm, comprising the steps of: melting an alloy in a carbon crucible, said alloy comprising a copper matrix of high purity copper with a total unavoidable impurity content of not more than 10 mass ppm and, contained in the matrix, 0.05 to 0.9 mass % of at least one metallic element selected from the group consisting of tin, indium, silver, antimony, magnesium, aluminum, and boron; and casting the molten alloy by means of a carbon mold.
In this production process, preferably, the casting is carried out by continuous casting to form a wire rod which is subjected to primary wire drawing, annealing, and then secondary wire drawing.
The production process according to the third feature of the invention can provide ultrafine copper alloy wires having high tensile strength and high electrical conductivity and, in addition, good drawability.
According to the fourth feature of the invention, there is provided an electric wire comprising a plurality of ultrafine copper alloy wires stranded together, said ultrafine copper alloy wires each having been drawn to a diameter of not more than 0.08 mm and being formed of an alloy comprising a copper matrix of high purity copper with a total unavoidable impurity content of not more than 10 mass ppm and, contained in the matrix, 0.05 to 0.9 mass % of at least one metallic element selected from the group consisting of tin, indium, silver, antimony, magnesium, aluminum, and boron.
According to the fifth feature of the invention, there is provided an electric wire comprising a plurality of ultrafine copper alloy wires stranded together, said ultrafine copper alloy wire comprising: a core wire formed of an alloy and drawn to a diameter of not more than 0.08 mm, said alloy comprising a copper matrix of high purity copper with a total unavoidable impurity content of not more than 10 mass ppm and, contained in the matrix, 0.05 to 0.9 mass % of at least one metallic element selected from the group consisting of tin, indium, silver, antimony, magnesium, aluminum, and boron; and, provided on the periphery of the core wire, a tin plating, a silver plating, a nickel plating, a tin-lead solder plating, a tin-copper-bismuth-base plating, or a tin-silver-copper-base lead-free solder plating.
The fourth and fifth features of the invention having the above respective constitutions can provide electric wires using ultrafine copper alloy wires, wherein, despite the same outer diameter as the conventional electric wires, the number of wire cores is larger than that of the conventional electric wires.
According to the sixth feature of the invention, there is provided a micro coaxial cable comprising:
an inner conductor comprising a plurality of ultrafine copper alloy wires, according to the first or second feature of the invention, stranded together;
an insulation covering the inner conductor;
an outer conductor comprising a plurality of ultrafine copper alloy wires spirally wound on the insulation at predetermined pitches; and
a jacket as the outermost layer of the micro coaxial cable.
In this micro coaxial cable, the ultrafine copper alloy wire constituting the outer conductor is preferably one according to the first or second feature of the invention.
The reasons for the limitation of numeral value ranges as described above will be explained.
The total content of unavoidable impurities in the high purity copper is limited to not more than 10 mass ppm from the viewpoint of minimizing the amount of inclusions in the high purity copper.
The amount of the metallic element contained in the copper matrix in the high purity copper is limited to 0.05 to 0.9 mass %. When the amount of the metallic element contained in the copper matrix is less than 0.05 mass %, a tensile strength of not less than 700 MPa cannot be ensured. On the other hand, the amount of the metallic element is larger than 0.9 mass %, an electrical conductivity of not less than 70% IACS cannot be ensured.
The reason why the tensile strength of not less than 700 MPa is required is as follows. When the tensile strength is less than 700 MPa, due to the very small wire diameter, the wires cannot withstand the stress applied at the time of producing stranded wires or at the time of extrusion of an insulation, leading to a fear of wire breaking. Further, in this case, the bending fatigue lifetime is not likely to be satisfactorily high as conductors.
The reason why the electrical conductivity of not less than 70% IACS is required, is that, when the electrical conductivity is less than 70% IACS, the transmission loss is large at the time of the flow of a high frequency current.
The diameter of the ultrafine copper alloy wire after drawing is limited to not more than 0.08 mm. When the wire diameter is larger than 0.08 mm, even conventional materials can provide extrafine copper alloy wires which can satisfy a tensile strength of not less than 700 MPa and an electrical conductivity of not less than 70% IACS and, at the same time, have good drawability.
The material constituting the crucible and the mold should be a carbon, from the viewpoint of avoiding the inclusion of pieces peeled from the crucible and the mold in the molten metal and the cast material during melting and casting.